<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      color: red;
    }

    .box {
      color: blue;
    }

    #home {
      color: green;
    }
  </style>
</head>

<body>
  <!-- 在同一个HTML中id不要重复,应该是唯一的 -->
  <div>我是div1</div>
  <div class="box">我是div2</div>
  <div id="home">我是div3</div>
  <p class="box">我是p元素</p>
  <h2 id="home">我是h2标题</h2>

  <div class="box one"></div>
  <div class="box-one box-first"></div>
  <div class="box_one box_first"></div>
  <!-- 大驼峰/小驼峰 -->
  <div class="boxOne BoxFirst"></div>
</body>

</html>